Marconi TF2604 Electronic Voltmeter Calibration

[Andrewausfa]

I was lucky enough to pick one of these up recently and also came across a manual for it.

Searching back it seems several people have these instruments and have asked for the manual. WMEBill kindly provided it in part in this thread -

https://www.vintage-radio.net/forum/...64&postcount=2

However the calibration procedure is missing. My scanner is dead so I have detailed out the basic calibration tests below. The rest of the procedures require equipment that I'm guessing most of us mortals won't have, like sources of d.c. voltage down to 0.5%, low frequency source down to 0.25% etc etc. The wording is much as the manual. Be aware the chassis is DC isolated so the Common lead is 'ground' not the chassis. So supply voltages are wrt Common lead.

Hope this lot is useful to someone.

Andrew

Power Supply Voltages

The supplies from the power unit may be monitored from tags on board TM 8371.

Variable resistor RV22 sets the HT supply. The voltages should be approximately as shown below

LT2 measured between tag 43 and 44 6.6vac
LT1 measured between tag 54 and 55 -6.6vdc
HT measured between tag 57 and 56 350vdc

Before starting these tests check power unit voltages are as given above.

Input resistance
1) Disconnect the instrument from mains supply and set the Selector switch for D.C.+ and any range
2) Check the switch on the D.C. probe is set to 'v'
3) Connect the universal bridge between the Common lead and the instrument chassis; the reading should be between 40 and 60 MOhm. If it isn't check R32 and C8.
4) Connect the bridge between the d.c./ohm probe and the common lead; the reading should be between 99 and 109MOhm.

Common Zero
1) With the instrument still disconnected from the mains supply, set the instrument to the +30vdc range and connect the d.c./ohm probe to the common lead.
2) Set mechanical zero (sic. the meter zero) and turn the COMMON ZERO control (RV3) fully clockwise.
3) Connect the instrument to the mains supply, switch on and allow to warm up for 2-3 minutes.
4) Switch to the +300mv d.c. range and adjust the Common Zero pre-set (RV2 marked ~/0 on front panel) to bring the meter to approximately half scale.
5) Turn the COMMON ZERO control fully counter-clockwise and select the -300mv range. The meter reading should be greater than 100mv.
6) Set zero by adjusting the Common Zero pre-set with the COMMON ZERO control set mid-way and then by adjusting the COMMON ZERO control.

AC Zero
1) Connect the d.c./ohm probe to the Common lead, select +300mv d.c. and adjust the COMMON ZERO control to bring the meter pointer to zero.
2) Connect the a.c. probe tip to the Common lead and switch to a.c.
3) Adjust the AC ZERO control (RV1) to zero the meter.
4) If the AC ZERO control is near the end of its travel, or if it has insufficient cover, select a new value for R5.

Set CZ
1) Switch to CZ and the 300mv d.c. range, connect the d.c./ohm probe to the Common lead and set centre zero by means of the COMMON ZERO control.
2) Switch to the 30 v range and check that the meter pointer is zeroed on the centre mark. If it isn't, adjust RV21 (located inside the instrument) until correct.
3) Check that it is possible to set centre zero on all ranges with the use of the COMMON ZERO control.

Set Ohms
1) Set the volts range selector to Ohms, the ohms range selector to '100' and the d.c./ohm probe switch to ohms
2) Connect the Common lead to the probe and set zero by use of the COMMON ZERO control.
3) Disconnect the probe from the Common lead and check the meter read infinity; if it doesn't adjust the SET Ohms pre-set (RV4 on the front panel).

Grid current
1) Set the d.c./ohm probe switch to 'v' and connect the probe to the Common lead.
2) Switch to the +300mv d.c. range and set zero.
3) Check that the grid current can be backed off, by disconnecting the Common lead from the probe and adjusting RV13 (located inside the instrument) to rest zero.